1. Field of the Invention
This invention generally relates to a controlling device for a brushless synchronous motor which supplies a staircase current waveform having a low high harmonic frequency component to the synchronous motor from a plurality of unit converting devices, which are capable of being phase-controlled.
2. Description of the Prior Art
In the past, two typical systems have been employed as driving systems for a synchronous motor. In one system a current having approximately a sine waveform, which is generated by a sine wave cycloconverter, is supplied to the synchronous motor, whereby the motor is operated with little torque ripple. This system is used with relatively large capacity motors.
On the other hand, in the other system a rectangular current, which is generated by an inverter or a rectangular cyclo-converter, is supplied to the synchronous motor whereby the motor is operated within a broad frequency range. This system is used with relatively small capacity motors.
The former system is advantageous in that it is possible to smoothly operate the synchronous motor with little torque ripple because the current waveform approximates a sine wave. However, the frequency range of operation is narrow and as a result it is difficult to operate the motor at half of the frequency of the power source or lower.
On the other hand, the latter system has an advantage in that it is possible to operate the motor over a wide frequency range. However with this system the higher harmonic components in the current waveform are increased because the current waveform has a rectangular shape. As a result, smooth operation of the motor at low speeds is difficult due to the generation of torque ripple in the motor.